1. Field of the invention
This application relates to the field of patch antennas and more particularly to the field of directional patch antennas using multiple patch radiating elements to control the direction of a beam of radio frequency energy (RF) over a large scan volume.
2. Description of Related Art
Many applications, such as scanning Radar and communication with satellites in a low orbit, require that the orientation of an RF beam emitted in three-dimensional space be adjusted rapidly with respect to a stationary reference axis without physically moving the antenna. This can be implemented using a stationary array of antenna elements which are coupled to an RF signal source and can be individually controlled. The spatial orientation of the RF beam can be changed by adjusting the relative phase of the RF signal supplied to the antenna elements. An antenna of this type is generally referred to as an "electronically scanned array", a "phased array" or a "patch" antenna and is described, for example, in the commonly assigned U.S. Pat. No. 5,400,040 "Microstrip Patch Antenna" to J. P. Lane et al., which is incorporated herein by reference.
The array antenna can either be assembled from individual antenna elements, or radiators, that are mounted on a passive support structure to form an array. The radiators represent individual waveguide cavities that terminate in a waveguide aperture; the waveguide apertures are typically co-planar with a ground plane. This approach minimizes the number of elements required for a desired array aperture and scan volume and maximizes scan volume coverage. On the other hand, the radiating aperture does not utilize the entire surface area of a "unit cell" since the area on the support structure located between the waveguide apertures is taken up by the ground plane, limiting the bandwidth of the device. Such antennas are also expensive to manufacture since each antenna element has to be inserted separately in the support structure.
Other known patch antennas are configured as a stacked patch, with each antenna element including a feed patch coupled to an RF signal source and a coupled patch separated from the feed patch by a dielectric layer, as illustrated in FIG. 1. Patch antennas of this type can be produced inexpensively by conventional integrated circuit manufacturing techniques, e.g., photolithography, on a continuous dielectric substrate. They have excellent frequency bandwidth since the radiating aperture is essentially the entire unit cell. Scan volume performance, however, is impaired due to the excitation of electromagnetic surface waves in the dielectric substrate. Surface wave excitation is especially severe when the dielectric constant of the substrate material is high, e.g., with advanced ceramic materials such as Low-Temperature Co-fired Ceramics (LTCC). It is therefore desirable to improve the antenna performance by eliminating or at least reducing the excitation of surface waves within the dielectric substrate.